The present invention relates to a shift control apparatus for an automatic transmission and more particularly to a shift control apparatus to govern a shift operation to effect a replacement gear shift by engaging a first friction engagement element by increasing a first fluid pressure and instead by disengaging a second friction engagement element by decreasing a second fluid pressure within a predetermined time interval in response to a pressure signal related to the first fluid pressure of the first friction engagement element.
An automatic transmission is configured to decide a drive path (or speed) of a toque transmission train by selectively actuating (engagement) friction engagement elements such as a plurality of clutches and brakes and to effect a gear shift by switching a friction engagement element to be engaged.
The automatic transmission effects a so-called replacement shift operation by engaging a first friction engagement element by increasing a first fluid pressure and instead by disengaging a second friction engagement element by decreasing a second fluid pressure. In this description, a friction engagement element to be changed from the engaged state to the disengaged state in the replacement shift operation and the operating fluid pressure are referred to as a disengagement side friction element and a disengagement side fluid pressure, respectively. A friction engagement element to be changed from the disengaged state to the engaged state and the operating fluid pressure are referred to as an engagement side friction element and an engagement side fluid pressure, respectively.
A published Japanese patent application Publication (Kokai) No. 5(1993)-157167 show a conventional shift control apparatus to control a replacement shift operation to effect a gear shift by engaging the engagement side friction element by increasing the engagement side fluid pressure and instead by disengaging the disengagement side friction element by decreasing the disengagement side fluid pressure in a manner to prevent a shift shock.
In the above-mentioned apparatus, a pressure regulating device holds the engagement side fluid pressure at a predetermined level until the engine speed (input speed) reaches a predetermined speed, and then increases the engagement side fluid pressure in accordance with the engine torque (input torque), so as to prevent a sudden change in the input speed, to vary the torque shares between the engagement side friction element and the disengagement side friction element smoothly and specifically to reduce a shift shock on a downshift.
An object of the first invention is to provide a shift control apparatus and process to vary a fluid pressure adequately to prevent shift shock when the shift operation is terminated forcibly.
According to the present invention, a shift control apparatus for an automatic transmission comprising first and second friction engagement elements to be selectively engaged and disengaged to select one of gear ratios, comprises at least a shift controller configured: to govern a first shift operation to effect a first gear shift by engaging the first friction element by increasing a first fluid pressure and instead by disengaging the second friction element by decreasing a second fluid pressure; to measure a time from a start of an inertia phase in the first gear shift; to check if an end of the inertia phase is not detected within a predetermined first time interval from the start of the inertia phase; and to forcibly increase the first fluid pressure at a predetermined gradual rate if an end of the inertia phase is not detected within the predetermined first time interval from the start of the inertia phase.